This invention relates to photoelectric element and device used for converting light energy into electric signals in the picture-reading apparatus such as a facsimile apparatus, more particularly, the invention relates to an improved photoelectric element which has a small dark current when in a reverse-bias state and, moreover, a high photoresponse rate, and which is readily and economically produced and can be produced in long lengths. The invention also relates to a device including the photoelectric element according to the invention and further to a process for producing the photoelectric element.
Photoelectric elements which have small dark currents in the state of reverse bias and, moreover, have high photoresponse rates are being widely used in picture-reading apparatuses such as those of facsimiles, and much research is being carried out with the aim of improving the performances of these photoelectric elements.
As photoelectric elements of this character, silicon-based photoelectric elements such as charge-coupled devices, metal-oxide semiconductors, and the like have heretofore been widely used. In the production of a silicon-based photoelectric element of such a character, a high degree of precision and fabrication technique is required.
More specifically, in the preparation of a silicon crystal, if there should be any ununiformity in the temperature distribution during its growth, the growth rate will fluctuate slightly, and segregation of the added active impurities will occur. As a consequence, ununiformity of the resistances in the diametrical direction and the axial direction will be produced. Furthermore, also in the formation of an oxide film for functioning as an insulating film in this element, it is necessary to rigidly regulate the temperature and the atmosphere, and, in the case where ions such as Na.sup.+ are contained as an impurity, moreover, there is the undesirable possibility of the characteristics of the formed product becoming unstable. In addition, in the etching of a substrate with an oxide film or the like as a mask, also, the etching rate varies with variations conditions such as the composition, temperature, and quantity of the etching solution and the rate of agitation, whereby it is necessary to strictly control these conditions.
Thus, a silicon-based photoelectric element is easily affected by the production process. As a consequence, deviations arise in the characteristics of the element, and the yield of the product is lowered, whereby the production cost becomes disadvantageously high.
Still another problem associated with silicon-based photoelectric elements is that, in the present state of the art of production thereof, it is difficult to fabricate elements of long lengths or large areas. For this reason, it becomes necessary to reduce the size of the original picture by means of lenses, and an optical system occupying much space becomes necessary, whereby it is difficult to reduce the size of the apparatus.